1. Field of the Invention
The present invention relates to a driving device for performing drive and control of so-called totem-pole-connected two switching devices and a technique for preventing a malfunction or incorrect action when a node or connected point between the two switching devices varies in potential.
2. Description of the Background Art
Inverters are heavily used for power devices for driving motors, for example. An inverter is powered by a power supply of a DC voltage of several hundred volts obtained by rectifying AC voltage and has a two-phase or three-phase configuration having a circuit as a unit in which two identical power switching devices are connected in series (totem-pole-connected) to the power supply. To cause the inverter to exercise a desired inverting function, the switching devices need to be switched (turned on/off) by a driving device in a correct order. ON command and OFF command for switching are low voltage signals of about several volts outputted from a drive control circuit or the like.
The low voltage signals can be supplied as they are to one of the switching devices on the low potential side, but cannot be supplied to the other switching device on the high potential side unless a reference potential is increased in level. This is because a circuit for switching the high-potential side switching device in the driving device operates at the potential of a node of the high-potential side switching device and the low-potential side switching device serving as a reference potential.
Thus, a photocoupler has conventionally been used widely to transmit low voltage signals to the high-potential side switching device, which, however, results in a considerable increase in costs. Further, research and development has recently advanced for integrating switching devices as well as their control circuits on the same semiconductor chip, in which the photocoupler has become a bottleneck in integration.
To solve such disadvantage, there is known a technique for transmitting ON and OFF commands to the high-potential side switching device through a level shift circuit.
However, such level shift circuit is provided across both circuits for switching the high-potential side and low-potential side switching devices. Thus, when the node of the both switching devices varies in potential with switching of these switching devices, noise resulting from this potential variation induces a malfunction or incorrect action of the level shift circuit (therefore, that of the driving device), resulting in a malfunction or incorrect action of the inverter.
An object of the present invention is to provide a driving device capable of preventing a malfunction or incorrect action even if a node of two switching devices varies in potential.
According to the present invention, the driving device performs drive and control of at least a high-potential side switching device of two switching devices connected in series between a first potential and a second potential higher than the first potential. The driving device includes a control signal generator, a level shifter, a dummy circuit, a mask circuit and a discriminator. The control signal generator is configured to generate a control signal for a conducting command to bring the high-potential side switching device into a conducting state and a non-conducting command to bring the high-potential side switching device into a non-conducting state, thereby outputting the control signal from an output terminal. The level shifter is connected to the output terminal of the control signal generator and configured to level-shift the control signal with at least one level shift circuit to generate a level-shifted control signal. A node between the two switching devices has a third potential. The at least one level shift circuit and the dummy circuit each include a current path provided between the first potential and a fourth potential set higher than the third potential, and a first switching device having a main path provided on the current path and a control terminal controlling conducting/non-conducting state of the main path. The control terminal of the first switching device in the at least one level shift circuit is connected to the output terminal of the control signal generator while the first switching device in the dummy circuit is always set at the non-conducting state. The at least one level shift circuit includes a first node outputting the level-shifted control signal while the dummy circuit includes a second node corresponding to the first node. The mask circuit is connected to the first node of the at least one level shift circuit and the second node of the dummy circuit and configured to mask a signal outputted from the first node using a signal outputted from the second node to generate a masked signal. The discriminator is configured to discriminate between the conducting command and the non-conducting command on the high-potential side switching device using the masked signal.
When the third potential varies with switching of the two switching devices, currents flow through the current path of the level shift circuit and that of the dummy circuit, and signals resulting from the currents are outputted from the first node of the at least one level shift circuit and the second node of the dummy circuit. Since the first switching device of the dummy circuit is always set at a non-conducting state, the signal outputted from the dummy circuit is nothing but noise resulting from the variation in the third potential. Thus, the mask circuit can remove noise from the signal outputted from the at least one level shift circuit using the signal outputted from the dummy circuit. In other words, the mask circuit can obtain, as the masked signal, the level-shifted control signal from which noise has been removed. As a result, a malfunction can be prevented.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.